1. Field of the Invention
This invention relates to sensors for monitoring fluid flow and more particularly, to a flow sensor and to a monitoring system incorporating the same for monitoring the condition of steam turbine drain valves.
2. Description of the Relevant Art
In a steam turbine power generating system, drain valves are placed in the steam turbine casings, steam turbine piping and auxiliary piping to prevent water induction incidents. Water forms as a normal consequence of bringing a large turbine to load, and can also collect during turning gear and synchronous operation due to improper piping design and system malfunction. Water induction is a problem that poses a threat to the operation and safe maintenance of steam turbine generators. The likelihood of experiencing a water induction incident increases and becomes more critical as the turbines age and when they are used for cyclic and/or for shift operation. Malfunctions of the equipment in the heat cycle can cause such induction to occur at various locations.
A problem associated with drain valves is that they sometimes become plugged and block fluid flow, even when open. Another problem is that drain valves sometimes may stick and thus remain in a closed position when conditions demand that they should be in an open position, or visa versa. Thus, a need exists for a sensor which can detect the presence or absence of fluid flow through a drain valve.
Flow sensors are generally known and used to determine the presence or absence of fluid flow. U.S. Pat. No. 3,366,942--Deane, illustrates a differential temperature sensor, used as a flow stoppage detector. The sensor, or probe, comprises a pair of heat sensing probes with a heater probe thermally connected therewith. The sensing and heater probes are adapted for being introduced into a conduit through which a fluid may flow. The heater probe is spaced more closely to one than to the other of the sensing probes. In the absence of flow, the sensing probe closer to the heater probe is at a higher temperature than the other sensing probe; conversely, when a fluid flows past the probes, heat is conducted away from the heater probe and thus the temperature difference between the two sensing probes decreases, or disappears.
U.S. Pat. No. 3,898,638--Deane et al., i r such differential temperature sensor, having the same basic configuration as that of the earlier Deane U.S. Pat. No. 3,366,942 but represented to have an improved internal structure of the temperature sensing probes which affords increased accuracy of measurements. As noted therein, differential heating of the two temperature sensing probes by the heater probe may be accomplished in part by, for example, the heat shunt running between the heater probe and the more adjacent of the two temperature sensing probes; further, both convection and/or conduction in the medium at rest, and conduction in the shunt, serve to carry heat differentially between the probes.
Monitoring systems presently used to monitor the condition of a drain valve and employing differential temperature sensors are ineffective or insufficient for a number of reasons. One system, shown in FIG. 1, employs two temperature sensing thermocouples 2 and 4 to passively detect a clogged condition of drain valve 1. This is accomplished by placing thermocouple 2 upstream of the drain valve 1 and thermocouple 4 downstream of the drain valve 1 and in thermal contact with pipe 3. The drain valve 1 is determined to be open and thus permitting hot fluid from the turbine to flow therethrough if both thermocouples 2 and 4 read substantially the same temperature. Conversely, the drain valve 1 is determined to be closed if the thermocouple 2 reads high and thermocouple 4 reads low. Determination of open or closed status is made by controller 5 which compares temperature signals supplied via lines 2' and 4' from thermocouples 2 and 4, respectively. Open or closed status is then compared to selected valve position as indicated at the control station via line 1'. Another known monitoring system, illustrated in FIG. 2, emploYs a similar temperature differential sensing technique in which two thermocouples 6 and 8 are placed downstream of the drain valve 7 with thermocouple 6 in thermal contact with the pipe 3' and thermocouple 8 being somewhat thermally isolated from the pipe 3' to provide a reference temperature. Determination of open or closed status is made by controller 9 which compares temperature signals supplied via lines 6' and 8' from thermocouples 6 and 8, respectively. Open or closed status is then compared to a selected valve position as indicated at the control station via line 7'.
The monitoring systems of FIGS. 1 and 2 are both inadequate for a number, of reasons. For instance, long thermal reaction times are required following the change of state of a drain valve. During these periods, which may take up to 45 minutes, the monitoring systems must be turned off or disabled to prevent false alarms. Another problem is that both systems rely on hot fluid from the turbine and cannot function when the turbine is cold. Therefore, during a cold state of operation, the monitoring system must be turned off to prevent false alarms. Another important problem is that the systems of FIGS. 1 and 2 cannot identify the reverse flow of cold water through the drain valve during a cold start-up.
Yet another problem with the known monitors is that they are easily confused when interconnected drains are used which allow steam from one drain to reach the sensor of another drain which is valved closed. When this occurs, the closed drain valve will be diagnosed as being stuck open. Such false indications need to be avoided.
Currently available sensors and monitoring systems incorporating the same using differential temperature to detect fluid flow have failed to satisfy critical needs in the industry. Accordingly, there remains a critical need for improved sensors and monitoring systems for reliably and rapidly detecting malfunctions of drain valves in steam turbine installations, thereby to permit corrective actions for avoiding the potentially serious water induction incidents which otherwise may result.